FIELD OF THE INVENTION
The invention relates to a method for determining the injection time in a direct-injection internal combustion engine.
The diesel direct injection is controlled by a controller. it is known to split up the injection process into one or more pre-injections and into one or more main injections in order to optimize combustion characteristics with regard to exhaust gas emission and combustion noise. A drive time of the injectors that is required for a desired quantity to be injected into the cylinders is determined by a characteristic diagram. An important input variable for the characteristic diagram is an instantaneous pressure in a lead or in the pressure reservoir, frequently denoted as a common rail or simply as a rail, since this is detected by measurement. The pressure in the lead is assumed to be constant in known systems. That is to say, the drive time of the injectors is determined with the aid of the same pressure for the pre-injection and also for the main injection. However, the pressure between two sequential injections, for example between the pre-injection and the main injection, can be regarded as constant only as a coarse approximation, as measurements substantiate. Pressure fluctuations are produced in the injector and in the lead and give rise to a marked change in the injection quantity.